Plutonium etching



United States Patent 3,380,865 PLUTONIUM ETCHENG Morris Kolodney, New York, N.Y., assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Apr. 28, 1948, Ser. No. 23,862 5 Claims. (Cl. 156-18) This invention relates to a method of treating metals and more particularly to a method of etching plutonium metal compositions.

Plutonium is bright and similar to nickel in tone when it is freshly cleaned. It is, however, readily corroded by air even at atmospheric temperature, gradually darkening and eventually acquiring a powdered oxide surface. At slightly elevated temperatures the rate of corrosion is considerably increased and the green-yellow dioxide is rapidly formed. Although plutonium alloys or plutonium metal compacts containing a major proportion by weight plutonium, corrode at a slower rate the powdered oxide also forms on their surfaces. All of the corrosion prodnets of plutonium are extremely toxic. The corrosion also means that plutonium objects are not dimensionally stable and therefore cannot be conveniently used in nuclear reactors, neutron multiplying systems and the like, where such stability is desired. Plutonium is very expensive and therefore the loss of material due to corrosion is very costly. Because of the toxicity of the products, the lack of dimensional stability and the loss due to corrosion, it is imperative that oxidation be minimized and the product be contained. In addition, it is often desirable that the applied coating be capable of absorbing the alpha radiation from the plutonium so that fabricated pieces may be handled without excessive safety precautions. It is also desirable that such coating not undergo nuclear reaction with the alpha radiation. For example, it is desirable that the coating not be susceptible to an alpha-neutron reaction. Prior to the time of this invention, however, no method was known for satisfactorily coating plutonium.

The primary object of this invention is, therefore, to obtain a method for eificiently treating plutonium metal compositions so that they may be successfully plated.

A further object of this invention is to provide a method for etching plutonium, plutonium alloys, or plutonium metal compacts containing a major proportion by Weight plutonium, so that a uniform, smooth, fine-grained adhesive coating may be obtained.

Still further objects and advantages of this invention will be evident from the following description.

The objects of this invention are achieved by the process of treating the surface of the plutonium metal composition with an acid solution containing sulfate ions. More particularly, the plutonium metal composition is etched by immersion in a solution containing ammonium sulfate and sulfuric acid or ammonium sulfate and acid sulfate salts. After the plutonium metal composition has been etched by the method of this invention, it may be readily plated with a protective metal by electroplating, metal carbonyl decomposition, metal spray or the like.

It is desirable before etching to clean the surface of the plutonium metal composition thoroughly, as, for example, by rendering it anodic in a solution containing equal parts of ethylene glycol and orthophosphoric acid as disclosed in the copending application S.N. 23,861, filed Apr. 28, 1948.

The invention may be best described by considering its presently preferred embodiment given in the following example. This example is given for the purpose of illustration only and not for the purpose of limiting the spirit or scope of the invention.

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Example A foil of plutonium about 0.5 inch square and 0.1 inch thick and a stainless steel foil of about the same size are made the anode and cathode respectively, in an electrolyte consisting of 50 cubic centimeters of concentrated orthophosphoric acid and 50 cubic centimeters of ethylene glycol at room temperature in a Pyrex dish. A current of 0.1 ampere per square inch is applied for 5 minutes. The plutonium foil is removed and immersed in concentrated nitric acid for about 1 minute to remove any discoloration.

After this preliminary cleaning the plutonium foil is etched by immersion for from 5 to 10 minutes in an aqueous solution of grams per liter of ammonium sulfate and 3 cubic centimeters per liter of concentrated sulfuric acid which is at room temperature. After this immersion the plutonium surface is well and uniformly etched and it will take a smooth, uniform coating. If a dark film which appears to be a complex compound results on the surface of the plutonium during the etching, the foil is immersed in concentrated nitric acid for about 1 minute, followed by a repeat of the original cleaning step in which the object is made anodic in the mixture of orthophosphoric acid and glycol.

After the plutonium foil has been etched, it is then silver plated at room temperature with a current density of 0.1 ampere per square inch in a solution containing grams per liter of silver cyanide, 204 grams per liter of potassium cyanide, and 15 grams per liter of potassium carbonate. The silver plate that results is smooth, bright, fine-grained and uniform. It adheres well and has good wearing qualities.

It may be seen that there are many possible embodiments of the methods of this invention. For example, the container may be any material which is not attacked by the electrolyte and which does not absorb the radioactive plutonium solution. Stainless steel may be used. The surface of the plutonium metal composition may be thoroughly cleaned by other methods than rendering the plutonium object anodic in mixtures of orthophosphoric acid and ethylene glycol. it is not necessary to dip the plutonium in concentrated nitric acid unless a dark film results from the cleaning. The concentration and anode current density of the bath in the cleaning procedure may be varied depending upon the size and shape of the object which is being cleaned.

In the above example the etching was carried out by immersion in a solution containing ammonium sulfate and sulfuric acid. Acid sulfate salts, such as sodium acid sulfate, potassium acid sulfate, and ammonium acid sulfate, may be used instead of or in addition to the sulfuric acid. This chemical treatment is simple and direct and does not require the use of electrical equipment. It is thought that the ammonium sulfate acts as a bulfer. The sulfuric acid or the acid sulfate is necessary to assure an acidic solution. The concentration of the solution may be varied within rather wide limits, but that of the sulfuric acid or the acid sulfate cannot be increased too much or the rate and amount of etching may be unsatisfactory. As an example of the possible concentration variations, the ammonium sulfate may be varied from 50 to 200 grams per liter and the sulfuric acid may be varied from 2 to 10 cubic centimeters per liter. After the plutonium object is etched, the dark complex compound which sometimes forms may be washed off by a stream of water. It may also be removed by other concentrations of nitric acid than the concentrated nitric acid used in the example or by other solutions containing nitrate ions such as ammonium nitrate in orthophosphoric acid. The method of removing the film by immersing the plutonium object in concentrated nitric acid, or in ammonium nitrate-phosphoric acid solution followed by rendering it anodic in an ethylene glycolphosphoric acid bath is preferable. This method assures that the object will be clean and free of all surface impurities.

The method of plating the etched plutonium object may be varied. For silver plating current densities as high as 0.2 ampere per square inch may be used, but for thick deposits it is preferable to use no more than half this quantity. Other metals such as tin, cadmium, nickel, and indium may, of course, be plated on the plutonium in place of or in series with the silver plate. The clean, etched plutonium may also be plated .by thermal decomposition such as by the deposition of nickel from nickel carbonyl, by metal spray methods, or by other methods, or combination of methods.

It is to be understood that when plutonium metal compositions are mentioned in the specification or in the claims, pure plutonium, or plutonium alloys, or plutonium metal compacts containing a major proportion by weight plutonium is meant.

Because many embodiments of the method of this invention may be accomplished Without departing from its spirit or scope, it is to be understood that the invention is not to be limited except as indicated by the appended claims.

What is claimed is:

1. The method of etching the surface of a plutonium metal composition to obtain a platable surface which comprises immersing a plutonium metal composition until the surface is etched in an aqueous solution consisting essentially of sulfuric acid and at least 50 grams per liter of a soluble inorganic sulfate salt.

2. The method of etching the surface of a plutonium metal composition which comprises immersing a plutonium metal composition until the surface is etched in an aqueous solution consisting essentially of sulfuric acid and a substantial concentration of ammonium sulfate.

3. The method of etching the surface of a plutonium metal composition which comprises immersing a plutonium metal composition until the surface is etched in an aqueous solution consisting essentially of at least grams per liter of ammonium sulfate and at least 2 cubic centimeters per liter of concentrated sulfuric acid.

4. The method of etching the surface of a plutonium metal composition which comprises immersing a plutonium metal composition in an aqueous solution consisting essentially of from 50 to 200 grams per liter of ammonium sulfate and from 2 to 10 cubic centimeters per liter of concentrated sulfuric acid for a period of about 5 to 10 minutes until the surface is etched.

5. The method of etching the surface of a plutonium metal composition which comprises immersing a plutonium metal composition in an aqueous solution consisting essentially of grams per liter of ammonium sulfate and 3 cubic centimeters per liter of sulfuric acid for a period of 5 to 10 minutes until the surface is etched.

References Cited UNITED STATES PATENTS 1,751,959 5/1930 Gravell 252-142 1,326,585 12/1919 Feenie et al. 252-142 X 2,000,576 5/1935 Boller 41.42

OTHER REFERENCES Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 12, page 31 (1932), pub. by Longmans, Green and Co., London.

Seaborg: Chemical and Engineering News, vol. 23, No. 23,'page 2192, Dec. 10, 1945.

JACOB H. STEINBERG, Primary Examiner.

JOHN R. SPECK, M. L. BREWRINK, SAMUEL BOYD, Examiners.

R. L. CAMPBELL, A. P. KENT, Assistant Examiners. 

1. THE METHOD OF ETCHING THE SURFACE OF A PLUTONIUM METAL COMPOSITION TO OBTAIN A PLATABLE SURFACE WHICH COMPRISES IMMERSING A PLUTONIUM METAL COMPOSITION UNTIL THE SURFACE IS ETCHED IN AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF SULFURIC ACID AND AT LEAST 50 GRAMS PER LITER OF A SOLUBLE INORGANIC SULFATE SALT. 